1. Field of the Invention
The present invention relates to rifaximin polymorphic forms α, β and γ, the processes for their preparation and the use thereof in the manufacture of medicinal preparations.
2. Description of the Related Art
Rifaximin (INN; see The Merck Index, XIII Ed., 8304) is an antibiotic belonging to the rifamycin class, namely a pyrido-imidazo rifamycin described and claimed in Italian Patent IT 1154655, while EP 0161534 discloses and claims a process for its production starting from rifamycin O (The Merck Index, XIII Ed., 8301).
Both these patents generically describe the purification of rifaximin by crystallization in suitable solvents or solvent systems and summarily show in some examples that the resulting product can be crystallized from the 7:3 mixture of ethyl alcohol/water and dried both under atmospheric pressure and under vacuum. Neither information concerning the experimental conditions of crystallization and drying, nor any distinctive crystallographic characteristic of the obtained product are reported.
The presence of different polymorphs had not been ascertained and therefore the experimental conditions described in both patents had been developed with the aim of obtaining a homogeneous product having suitable purity from the chemical point of view, apart from the crystallographic aspects of the product itself.
It has now been unexpectedly found that some polymorphic forms of rifaximin exist whose formation depends on the solvent as well as on the conditions of time and temperature at which both crystallization and drying are carried out.